A large portion of the signal processing section of a modern television receiver is incorporated within a single integrated circuit (IC) sometimes referred to as a "one-chip" television IC. It is desirable that such ICs perform as many functions as possible to reduce the amount of external circuitry that is required. In addition to the "normal" signal processing functions, an IC often includes provisions for testing and/or aligning the IC and associated external circuitry. With the increased complexity of signal processing ICs, such as those employed in television receivers, it is frequently found that there are not enough IC terminals (commonly also referred to as "pins") available for all of the required functions. Accordingly, some terminals are used for dual purposes.
One common use of a "dual purpose" terminal is to receive an input signal which is used for signal processing during the "normal" operating mode of the IC and also to receive another input signal which is used to enable a "test" mode of the IC. It is intended that the amplitudes of the two input signals remain within two different ranges corresponding to the two modes of operation so that the signal intended for signal processing does not cause the test mode to be activated. This is readily accomplished when the two signals are each generated in a predictable manner.
The present invention is directed to the situation in which it may not be possible to ensure that one of the two input signals remains in its respective amplitude range. The present inventors have recognized that this situation may occur when the input signal which is associated with the "normal" signal processing operation is generated as a combination of signals produced by two or more circuits which operate more or less independently of each other.
By way of example, the TA8680 "one-chip" television IC available from Toshiba Corp. includes provisions for using the picture contrast control terminal to enable a test mode. It is intended that the amplitude of the contrast control voltage coupled to the contrast control terminal remain in a predetermined range for normal signal processing operations. When it is desired to enable the test mode, a test mode enabling DC voltage, well below the range of the contrast control voltage, is coupled to the contrast control terminal. This arrangement works quite well as long as the contrast control voltage remains within the required range. It may be possible to meet this criteria by properly selecting component values when the contrast control voltage is generated by a user contrast adjustment circuit. However, it has been recognized by the present inventors that for certain situations requiring one or more image content responsive beam current limiting circuits to be coupled to the contrast control terminal, it may not be possible to ensure that the contrast control voltage remains in the required range under all operating conditions. As a result, the test mode may be undesirably activated, causing a disconcerting image disturbance.